Light scattering by small particles is a key issue for several optical applications, such as imaging through turbid media or free space optical telecommunication (FSO) between ground based stations and satellites. Many approaches based on adaptive optics and spatio-temporal shaping of laser beams have been proposed for improving the propagation through scattering media and for avoiding speckle patterns formation [1]}, [2]}, [3]}, [4]}, [5]}, [6]}. These methods, however, are of limited use for dense and extended structures such as atmospheric fog and clouds. For free space optical networks, this critical issue is currently mitigated by the multiplication of transmitting/receiving stations, which increases complexity and cost. Until recently no active method for opening clear optical transmission channels through clouds or fog was available. The propagation of ultrashort and high intensity lasers through the atmosphere provided a radically new concept, based on plasma generation inherent to laser filamentation [7]}, [8]}, [9]}. Laser filaments are self-sustained light structures with a typical core diameter \(d_{core}\sim 100\,\mu \) m (at 800 nm) and up to hundreds of meters in length, orders of magnitude beyond the linear diffraction limit. They stem from the dynamic balance between self-focusing (Kerr effect) and defocusing (diffraction, plasma-induced refraction, etc.) that takes place during non-linear laser propagation in air [10]}. Filaments bear high intensities (\(10-100\)  TW/cm\(^2\) ) and generate a low density plasma during propagation. The plasma recombination thermalizes quickly, resulting in a very rapid local increase of pressure that drives a single cycle cylindrical acoustic wave [11]}, [12]}, [13]}, [14]}, [15]}. Over a time window of \(0.1-1\)  ms, this leads to a reduced air density channel significantly wider than the filament core [13]}, [14]}, [15]}, [19]}, [20]}. When launched in fog (water droplet diameter centered around 5 \(\mu \) m), this filament-induced shockwave radially ejects the scatterers out of the beam [8]}, [9]}. This fog clearing mechanism requires modest energy deposition, of the order of less than 1 mJ/m [23]}, [24]}. For droplets densities \(N_d \sim 10^5\,\) cm\(^{-3}\) , it was shown that the filament formation is not significantly altered [25]}. This is the case in the vast majority of scenarios in the field [25]}, [27]}, [28]}, and is the case for our fog chamber as discussed below. Under these conditions, there is little additional energy cost due to this droplet clearing mechanism, as the energy deposition and acoustic wave generation through plasma generation by the filament occurs in any case as a consequence of its propagation.
